1. Field of the Invention
This invention relates to a method for reducing the volume of off-flow gases created in the production of steel by the Basic Oxygen Steel Process. The making of steel using the Basic Oxygen Steel Process involves the use, typically, of a furnace or vessel of a size that can hold between fifty and four hundred tons of steel. Initially the furnace is prepared with a molten metal charge, e.g., pig iron, which contains carbon impurities of one to five percent. Additionally, cold or scrap metal is conventionally added to the furnace, followed by slag-forming material which forms a layer covering the surface of the metal charge.
Operation of the Basic Oxygen Steel Process results in reduction of the amount of carbon impurities within the metal charge to a level which is satisfactory for the desired steel. The amount of these carbon impurities is reduced by introducing oxygen into the furnace and allowing it to react with the carbon to form carbon monoxide and carbon dioxide. The introduction of this oxygen is often effected by the lowering of a water-cooled oxygen lance into the oxygen furnace to a point just above the metal charge. Oxygen is then blown through the lance at a relatively high rate which results in a blowing aside a portion of the slag layer, thereby allowing the oxygen to interact directly with the metal charge.
The reaction of carbon and oxygen to form carbon monoxide and carbon dioxide is exothermic, generating large amounts of heat energy, and in the Basic Oxygen Steel Process is manifested by a large flame cone which contains carbon monoxide, carbon dioxide, dust particles and other impurities which escape through the opening of the furnace at extremely high temperatures as off-flow gases. While this process for making steel is very efficient and has supplanted, and is continuing to supplant, conventional open-hearth furnaces, the amount and character of the off-flow gases is a cause of concern, and because of the present environmental requirements, substantial effort is being expended to properly control these off-flow gases. Presently, methods are being used to confine the off-flow gases and treat them in a manner that reduces their toxicity to a point compatible with the environment, as by oxidizing the carbon monoxide to carbon dioxide for later release into the atmosphere, where air is used as the oxidizing medium.
2. Brief Description of the Prior Art
Use of the Basic Oxygen Steel Process in facilities which do not provide for the collection of these off-flow gases, or do not flare the off-flow gases, require a large exhaust system to handle the large volume of these off-flow gases. Typically, these off-flow gases are emitted from the furnace at extremely high temperatures and with relatively high concentrations of carbon monoxide. The conventional exhaust systems include a large exhaust fan fo draw the off-flow gases into and through the exhaust system and draw in large amounts of air, thus facilitating oxidation of the carbon monoxide to carbon dioxide, as well as cooling the off-flow gases.
In a representative embodiment of the Basic Oxygen Steel Process, 12,500 standard cubic feet per minute (SCFM) of oxygen is needed to oxidize the carbon monoxide produced from a 100 ton oxygen furnace which contains 30% scrap and 70% hot metal charge with a 4% carbon impurity content. In a system of this nature, approximately 13,100 SCFM of carbon monoxide, 1,540 SCFM of carbon dioxide and 760 SCFM of other miscellaneous gases such as argon, N.sub.2, and SO.sub.2, are generated. Further, the exhaust system draws air in through the gap between the exhaust hood and the opening of the oxygen furnace in amounts of approximately 121,850 SCFM, this volume of air varying, depending upon the amount of cooling necessary and the exact configuration of equipment used within the exhaust system.
The final volume of gas which must be collected by the exhaust system in the above representative embodiment, is on the order of 202,000 SCFM, which is comprised of:
13,100 SCFM of carbon monoxide oxidized to carbon dioxide PA1 1,540 SCFM carbon dioxide from the initial oxidation PA1 760 SCFM of miscellaneous gases PA1 116,300 SCFM of air drawn in by the exhaust system (5,550 SCFM of the oxygen within the air is used to oxidize the carbon monoxide) PA1 70,300 SCFM of cooling water.
That amount of cooling water is required to lower the temperature of the off-flow gases so that they can by appropriately handled by the environmental conditioning equipment within the exhaust system, and the large amount of air required by the exhaust system is due, in part, to the fact that approximately 78% of air is nitrogen, and not relevant to the oxidation process.